Genuine U.S. paper currency contains a variety of printed indicia which may be used to identify the currency as authentic, and also to distinguish between authentic currency of various denominations.
One indication of authenticity is the fact that certain areas on a U.S. bill are printed with ink with magnetic properties. For example, the portrait which appears in the center of every U.S. bill is, in a genuine bill, printed entirely with magnetic ink. The fanciful engraving which forms the printed border of each U.S. bill is likewise composed entirely of magnetic ink, as are the large capital letters or large numerals which appear to the right of the portrait and which identify the denomination of the bill (i.e., "ONE", "TWO", "FIVE", etc.). In contrast, the green Treasury Department seal which underlies the denomination identifying letters or numerals to the right of the portrait, as well as the black Federal Reserve Bank seal which appears to the left of the portrait, are both printed in non-magnetic ink.
Each denomination U.S. bill is likewise characterized by the distance between the grid lines which comprise the background of the portrait field. In one dollar bills, for example, the space between vertical grid lines is equal to 0.008 inches. For two and five dollar bills, the grid line space is equal to 0.010 and 0.011 inches, respectively.
Prior art currency validators have been proposed which identify authentic U.S. bills and distinguish between bills of various denominations by measuring the average spacing between the vertical grid lines in the portrait areas of the bills. One such device is disclosed in U.S. Pat. No. 4,349,111, Shah et al.
Identification of bills based on average grid line spacing is likely to lead to failures to distinguish between bills having relatively small differences, in grid spacing. For example, certain commercial bill validators utilizing the average spacing technique cannot be used with both two dollar and five dollar bills, because the average grid line spacings are too similar.
Another problem with various prior art validators is that they may accept high denomination bills as valid lower denomination bills.
Many prior art current validators require that the tested bill be inserted into the validator in. a specific orientation (e.g., Federal Reserve seal first). Such devices result in authentic bills being rejected merely because of improper orientation.
Many of the prior art currency validators require careful regulation of the speed at which the bill is scanned for information. In such validators, even a slight variation in scanning speed, such as that resulting from an instantaneous drop in power line voltage, can cause authentic bills to be rejected and produce inaccuracies in the identification of bill denomination. It is therefore desirable to provide a currency validator which is insensitive to the speed at which a bill is scanned.
In order to avoid some of the problems of speed regulation, some prior art validators, such as disclosed in U.S. Pat. No. 4,464,787, Fish et al., employ detectors at fixed positions by positively identifying the position of the bill and thereby ascertain the bill area being tested. These validators, however, generally require a testing channel at least as long as the bill being tested.
In recent years, the high quality of copying and printing machines has resulted in a serious counterfeiting problem. One of the countermeasures against counterfeiting which has been adopted in many countries is the use of a metal embedded in paper checks or currency notes. However, metal detectors for automatically discriminating between genuine and counterfeit currency notes and checks by detecting the metal strip have not been put to practical use because they have not been capable of accurate and reliable detection of the metal strips at high speed.
An image (on a bill) can be scanned with a resolution of 300 or 600 dots per inch and the stored imager printed using inkjet printers that can match virtually any color.
The present invention overcomes the aforementioned prior art problems and provides a device for accurate detection of counterfeit currency at very high speeds.
Experience has shown that there is a large difference in frequency dependent, electrical complex permittivity between genuine and counterfeit bills. Genuine bills have a substantially different complex permittivity which is distinct from the permittivity of counterfeit bills. The invention broadly measures the complex permittivity, .epsilon., of the `paper` bills to determine if the bill(s) is genuine or counterfeit.
The invention comprises a device and method for determining the genuineness of a bill(s) by measuring the complex permittivity based on the frequency dependent dielectric properties of the bill(s).
The complex electric permittivity of a dielectric material is typically read by measuring complex impedance using a capacitor to sense material properties. There are a wide variety of techniques known in the art for measuring capacitance including RF bridge circuits, oscillators, etc. Preferably the permittivity is sensed as it affects the complex impedance. This may be expressed as it affects the resonant frequency of an oscillator such as a Wein bridge or phase shift oscillator. Electrodes are placed in electrical communication with the bill to be tested. The electrodes may be on opposite sides of the bill or using an interdigitated pattern the sensing electrodes may be on the same side. If interdigitated electrodes are used to sense the permittivity of the bill, the spacing between the electrodes is set so that the fringing electric field permeates the bill, typically 0.004" thick, but does not significantly penetrate the material beneath the bill, i.e. the support surface.
When the sensing electrodes engage the bill, a capacitor is formed. This is due either to the field penetrating the bill between electrodes on opposite sides or due to the fringing field. The value of the capacitance is strongly affected by the electric permittivity of the bill. The capacitor forms part of an oscillator such as a Wien bridge oscillator or a phase shift oscillator, although any oscillator circuit will function as long as its resonance frequency depends upon the value of a capacitor and that capacitor can be formed by the electrode-currency combination. Moisture will affect the surface conductivity of the bill. This can be accounted for by modelling the bill as a capacitor in parallel with a surface resistance. Moisture effects will be less pronounced when using electrodes on each side of the bill.
In a particularly preferred embodiment of the invention, the output frequency from the oscillator is fed to a tone decoder, such as a phase locked loop circuit. The phase locked loop has a frequency capture range. The voltage output of the tone decoder is one of two levels. Level one is realized if the oscillator frequency is within the capture range. Level two is observed if the frequency is outside the capture range. The tone decoder can be configured so that the oscillator frequencies observed for genuine bills lie within the capture range producing a positive output while those for counterfeit bills are outside the capture range and produce a negative output.
It is also within the scope of the invention that the complex electric permittivity of the currency be sensed at a plurality of frequencies. At each of these frequencies, the real and imaginary impedance is measured. These measurements (readings) are compared against a standard, corresponding to genuine currency, which may include using discriminant analysis techniques. An output indicates whether the bill is counterfeit or genuine. Several models can be used to sense the electric permittivity of the currency, i.e. a total impedance model, a voltage divider model or a bridge model. The bridge model can comprise two similar arms A and B, each of which is a parallel combination of a resistor and a capacitor. The parallel combinations of these arms are separately in series with a resistor forming arms C and D. The bridge voltage is read between the nodes at the junction of arm A and arm C and the junctions of arms B and D .